1. Field of the Invention
The present invention relates to an information recording medium employable for optically recording and reproducing a desired information.
2. Description of Prior Art
Information recording media utilizing a beam of high energy density such as a laser beam have been developed in recent years and are now put into practical use. Such recording medium is generally called "optical disc" and the practical applications thereof have been found, for example, in a video disc and an audio disc as well as in a disc memory for a large-capacity computer and a large-capacity static image file.
The optical disc basically comprises a disc-shaped transparent substrate of plastic material or glass material and a recording layer provided thereon which is made of a metal such as Bi, Sn, In or Te or a semi-metal. Writing of information on the optical disc is conducted, for example, by irradiating the disc with a laser beam. The irradiated area of the recording layer of the optical disc absorbs energy of the beam, and rise in temperature locally occurs. As a result, a physical change such as formation of a pit or chemical change such as phase change is caused to alter optical characteristics of the recording layer in the irradiated area, whereby the recording of information is made. Reading of information from the optical disc is also conducted by irradiating the disc with a laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the alteration of the optical characteristics of the recording layer.
The optical disc is desired to have high durability as well as improved recording and reproducing characteristics. For the reason, studies for improving the recording layer have been made.
As a material employable in the recording layer, Te (tellurium) has been paid attention owing to its particularly high sensitivity in the recording procedure. However, such high-sensitive Te has the drawback in that Te is readily oxidized in contact with ambient atmosphere.
For the reason, Te is generally used in combination with other metals as mentioned below mainly to prevent Te from its oxidation and to improve the durability. For example, there have been known a recording layer composed of Se and Te (Japanese Patent Publication No. 59(1984)-35356), and a recording layer containing Te or Bi and Se (Japanese Patent Provisional Publication No. 57(1982)-38189).
Further, Japanese Patent Provisional Publication No. 57(1982)-66996) discloses a recording layer composed of a composition of Se, Te and metal selected from In, Pb, Sn, etc, which has the formula of Se.sub.a Te.sub.b M.sub.c, wherein a, b and c are values in the ranges of 0.02.ltoreq.a.ltoreq.0.35, 0.50.ltoreq.b.ltoreq.0.98 and 0.ltoreq.c.ltoreq.0.45, respectively, and M is one metal selected from the group consisting of Pb, In, Ge, S, Sb, As, Bi, Sn, Al, Ga, Tl, Zn, Cd, Au, Ag, Cu, Ni, Pd, Rh, Cr, Mo, W and Ta.
To improve the durability, it has been also proposed to provide a protective layer of tellurium oxide on the surface of a recording layer composed of Te from the viewpoint of protecting the recording layer (Japanese Patent Provisional Publication Nos. 58(1983)-94144, 58(1983)-189850 and 61(1986)-142541). However, although the provision of the protective layer of tellurium oxide is effective to improve durability of the tellurium recording layer, the tellurium oxide layer likely brings about deterioration of the recording characteristics of the recording layer, because recording sensitivity of tellurium oxide is relatively low.